A crane truck is known from DE 39 12 868 C1, the superstructure of which has a counterweight plate on the counterboom for the boom, which is retained on the counterboom of the superstructure for reasons of load compensation and stability during crane operation of the crane truck. During driving operation the counterweight plate is placed on a support on the undercarriage preferably between the driver's cabin of the crane truck and the latter's superstructure, whereby improved load distribution and relief of the slewing rim in attained.
Lowering and lifting of the counterweight plate takes place by means of an elevating mechanism consisting of two vertical hydraulic lift cylinders on the counterboom. The two piston rods of the lift cylinders are drivable to rotate about their longitudinal axis between a locking position non-positively connected with the counterweight plate and an unlocking position released from the counterweight plate. For regular performance of the unlocking rotation the piston rod with its locking element--for instance a transverse bolt at the free end of the piston rod--must be in a so-called piston-rod-release position, in which the non-positive connection between the piston rod and the counterweight plate is released.
With the known crane truck problems may arise regarding the mounting of the counterweight plate to the counterboom by means of the elevating mechanism and regarding the change-over of the crane truck from crane operation into driving operation. For instance, the counterweight plate may come out of its regular crane-operating position during crane operation due to a defect in the hydraulics of the elevating mechanism and move downwards, which cannot or can only hardly be noticed by the crane driver sitting in the driver's cabin on the superstructure during crane operation with his back turned to the counterboom with the counterweight plate. Upon rotation of the superstructure the counterweight plate in its improper lowered position may run against an obstacle, which may cause considerable damage to the crane truck on the one hand and to the obstacle on the other hand. Also the stability of the crane truck during crane operation may be impaired.